This invention relates to apparatus for use in scanning a cinematograph film from which a television video signal is to be produced while the film is being moved in a continuous manner.
The principal problem which such apparatus must cope with is the difference between the rate at which conventional cinematograph film must be viewed in order to give a reproduction of the appearance of the recorded scene which is true with regard to movement in the original scene, and the conventional rate of formation of television pictures. For example, many cinematograph films are intended to be projected at 24 frames per second, and in the European PAL system of colour television, 50 television fields consisting of 25 pairs of interlaced rasters are formed each second. It is well known to run such a film at 25 frames per second, since at this rate there is substantially no noticeable unnaturalness about movement in a reproduced scene, and to scan each film frame picture twice. The pair of rasters which can each frame picture in this method are displaced from one another, the second of the pair being displaced from the first in the direction of movement of the film. Furthermore, for a conventional cinematograph film in which the intended horizontal dimension of each film frame picture is transverse to the length of the film, the vertical dimension of each raster is compressed in proportion to forward film speed and expanded in proportion to reverse film speed.
The conventional measures adopted to allow for these effects of the film movement on the vertical extent needed for each raster and the vertical positioning of the raster are to generate a basic vertical deflection signal in the form of a regular sawtooth with a repetition rate equal to the field frequency of the television system, e.g. 50 hertz for PAL, and a slope which would give the required vertical deflection of a cathode ray tube generated raster pattern for projection onto a stationary frame picture of the film, and to superpose this basic signal on a compensation signal which is derived by sensing the film speed and direction and the position of film frames as they enter a scanning region. In a co-pending patent application entitled "Apparatus for use in scanning a cinematograph film", by Andrew S. Armstrong and Martin R. Sach, there is described apparatus for use in scanning a cinematograph film from which a television video signal is to be produced while the film is being moved in a continuous manner in a film transport, the apparatus including movement signal generating means responsive to movement of a cinematograph film in the transport to produce a signal indicative of direction of movement of the film and a position signal having a value representative of the position of a predetermined point along the length of each film frame during the time in which the said point moves from a reference position in a film scanning region to a position reached when the predetermined point of the next film frame entering the scanning region arrives at the reference point. Raster reference means produce a raster reference signal having a value representative of the rate of increase of the vertical coordinate of a scanning raster of the dimensions required for scanning a frame of the film in the absence of movement of the film. Signal combining means are coupled to the movement signal generating means and to the raster reference means and adapted to sample the value of the position signal substantially at each time at which a scanning raster is to be initiated by an electro-optical raster source when coupled to a raster output of the combining means. The combining means are supplied by the movement signal generating means with the signal indicative of direction of movement and with a signal representative of rate of movement of the film and produce at the raster output in each interval beginning substantially at the time of a respective sampled value of the position signal and ending substantially at the time of the next sampled value thereof an output signal having a value substantially representing the said vertical coordinate of the scanning raster required for scanning a frame of the film during the said interval, the said vertical coordinate being parallel to the length of the film at least at the scanning region. In the apparatus specifically described therein, the movement sensing means is such that the said signal representative of rate of movement of the film varies with the length of film passing through the scanning region in a unit of time, and includes means for generating a reference train of pulses having a repetition rate which varies with but is not greater than the frame rate of the film. The reference train of pulses is generated having a repetition rate which is an integral submultiple of the frame rate of the film and is utilized in the production of a further reference train of pulses with a repetition rate equal to the frame rate. In this particular apparatus, the movement signal generating means includes an up/down counter arranged to be so reset by each pulse of the further reference train that the count in this counter at the start of each counting cycle is representative of the said reference position, the said signal representative of rate of movement of the film being supplied as clock signal to the said up/down counter, and the signal indicative of direction of movement being so supplied to the said up/down counter as to determined in direction of counting thereof in dependence upon the direction of movement of the film, whereby the count in the said up/down counter is representative of the position of the predetermined point of each film frame during the said time in which the said point moves from the reference position to the other said position.
For producing the said signal indicative of direction of movement of the film, the movement signal means includes means for generating two trains of pulses having equal repetition rates and a phase displacement relative to one another which changes only with change in the direction of movement of the film. The equal repetition rates of the said two trains are equal to an integral multiple of the frame rate of the film, and the movement signal generating means includes a pulse rate dividing counter adapted to divide the rate of one of the said two trains of pulses and to thereby produce the said further reference train of pulses, the dividing counter being arranged to be reset by each pulse of the first said reference train of pulses. The two trains of pulses having equal repetition rates are applied to a bistable circuit such that the state of the bistable circuit changes in response to a change in the relative phase displacement of the said two trains.
For use in so producing the said signal representative of rate of movement of the film that the said signal varies with the length of film passing through the scanning region in a unit of time, the movement signal generating means includes a pulse generator arranged to generate pulses at a rate dependent upon the rate of rotation of a capstan drawing film through the said scanning region.
Furthermore, the signal combining means includes an up/down counter adapted to load in the count in the up/down counter of the movement signal generating means substantially at each time at which a scanning raster is to be initiated and to count from the count thus loaded therein a plurality of clock pulses produced in the signal combining means in response to the said signal representative of rate of movement, and a plurality of clock pulses produced in the signal combining means in response to the said raster reference signal, the direction of counting of the clock pulses produced in response to the said signal representative of rate of movement being carried out in a direction determined by the signal indicative of direction of movement of the film and the direction of counting of the clock pulses produced in response to the said raster reference signal being arranged to correspond to the direction required for producing output signal values representative of the increasing vertical coordinate of a scanning raster for scanning a frame of the film in the absence of movement of the film.
The use of the respective said up/down counters in the movement signal generating means and the signal combining means with their respective directions of counting determined directly by the signal indicative of direction of film movement alone or in combination with a further, intermittent signal indicative of the direction for producing the increasing vertical coordinate of a scanning raster in the absence of film movement results in considerable complexity in ancillary circuitry for ensuring that the times of occurrence of the various clock and resetting signals do not result in mutual interference and the generation of spurious signals.
It is accordingly an object of the present invention to provide apparatus for use in scanning a cinematograph film in which the risk of mutual interference between signals is reduced and the ancillary circuitry is simpler.